The invention relates to a time-alignment apparatus of a receiver of a telecommunication system for receiving successive data frames on a plurality of channels. The respective data frames on said channels are not time-aligned to each-other and the time-alignment apparatus outputs data frames of all channels in a time-aligned manner with respect to a common synchronization clock. The invention also relates to a method performing such a time-alignment, a receiver of a telecommunication system and a telecommunication system in which such a time-alignment is performed in a receiver.
In many telecommunication systems the individual data frames of a number of channels are not received on the radio link at exactly the same time. For example, if the telecommunication system is a mobile radio communication system, then the distances between the base transceiver station (BTS) and the mobile stations vary among a number of mobile stations, even over time during a connection, such that the data frames arrive in a non-time-aligned manner.
An example of such a mobile radio communication system is a CDMA system as shown with the base transceiver station BTS in FIGS. 5, 6. Generally, the above described aspects with respect to the time alignment are also applicable to any telecommunication system using a plurality of non-aligned channels.
In all telecommunication systems where several channels each comprising successive data frames are provided, a separate decoder dedicated to a specific channel must be provided in order to decode the successively arriving data frame of only one particular channel. However, this would result in up to 300 decoder units, which is unacceptable in terms of the required hardware effort. For this reason, there is always the problem of how a common decoder resource can be used efficiently for decoding the data frames of all channels. In principle, this can be achieved if the time which the decoder spends on decoding one data frame is much shorter than the duration of a data frame. Then, the decoder can process the data frames of several channels one after each other within one data frame period. However, this requires the incoming data frames to be buffered in a memory before they can be delivered in form of a constant stream of data frames to the decoder resource. For this purpose the data frames must be arranged to fit a given time grid, i.e. they must be aligned with respect to the internal frame structure of the decoder, i.e. to a common synchronization clock provided internally in the receiver.
The invention particularly relates to the problem of how the different time offsets of the received data frames of a great number of channels can be handled, such that only one common decoder resource is necessary.
As described above, due to the fact that the data frames from different channels have individual time offsets to the internal frame structure of the decoder, the data frames must be aligned to the internal synchronization clock in a well defined manner (frame alignment) to make it possible to use the decoder resources in a time sharing manner. In addition, a de-interleaving must often be performed, i.e. the received data frames must be rearranged (de-interleaved), before the frame can be decoded.
The object of the invention is to provide a time-alignment apparatus, a receiver of a telecommunication system, a telecommunication system and a time-alignment method, with which the decoder resources can be used efficiently even for a large number of channels.
This object is solved by a time-alignment apparatus of a receiver of a telecommunication system for receiving successive data frames on a plurality of channels, wherein said respective data frames on said channels are not time-aligned, and for outputting said data frames of all channels time-aligned to a common synchronisation clock, comprising:
a) at least a first, second and third read/write frame memory for respectively storing one data frame of each of said channels, said frame memories each having a write state in which data is written to said frame memories and a read state in which data is read from said frame memories; and
b) a control unit for cyclically switching said three frame memories through a first to third alignment mode synchronized to said common synchronisation clock such that
b1) in said first alignment mode said first and second frame memory are in a write state and said third frame memory is in a read state;
b2) in said second alignment mode said second and third frame memory are in a write state and said first frame memory is in a read state;
b3) in said third alignment mode said third and first frame memory are in a write state and said second frame memory is in a read state;
b4) wherein after each mode switching a newly arriving data frame of any channel is always written to a frame memory which was in a read state in the previous mode; and
b5) wherein data frames are always read from the frame memory having a read state time-aligned to said common synchronization clock.
Furthermore, this object is solved by a receiver of a telecommunication system comprising one or more time-alignment apparatus.
The object is also solved by a telecommunication system comprising one or more receivers.
Furthermore, the object is solved by a method of successive data frames on a plurality of channels, wherein said respective data frames on said channels are not time-aligned, and for outputting said data frames of all channels time-aligned to a common synchronisation clock, comprising:
a) writing data frames into at least a first, second and third read/write frame memory for respectively storing one data frame of each if said channel, said frame memories each having a write state in which data is written to said frame memories and a read state in which data frames is read from said frame memories; and
b) cyclically switching said three frame memories through a first to third alignment mode synchronized to said common synchronisation clock wherein
b1) in said first alignment mode data frames are written to said first and second frame memory in a write state and data frames are read from said third frame memory in a read state starting with each common synchronization clock;
b2) in said second alignment mode data frames are written to said second and third frame memory in a write state and data frames are read from said first frame memory in a read state starting with each common synchronization clock;
b3) in said third alignment mode data frames are written to said third and first frame memory in a write state and data frames are read from said second frame memory in a read state starting with each common synchronization clock; wherein
b4) after each mode switching a newly arriving data frame of any channel is always written to a frame memory which was in a read state in the previous mode;
b5) wherein data frames are always read from the frame memory having a read state time-aligned to said common synchronization clock.
According to one aspect of the present invention, three frame memories are used. Each frame memory can hold one complete data frame of all channels. During one synchronization clock period two memories are used to write data in and one is used to read data out. Incoming data frames are always written to one of the frame memories in a write state when they are received. Since the time offset between the data frames of the individual channels and the synchronisation clock can vary between 0 and one whole frame period, it can, however, take up to two synchronisation clock periods, until one whole data frame from every channel is stored in one frame memory. Since it can take up to one entire frame period until the data frame, which has been fully written into a frame memory, is read out, and another data frame of the same channel will arrive directly after the pervious frame, a second memory in a write state is used.
Whilst writing the individual data frames of the plurality of channels to one of the two frame memories, the third frame memory (in the read-state) is read out. When the next common synchronization clock occurs, the alignment mode of the three frame memories is cyclically changed. That is, the frame memory used for reading in one alignment mode will then be used as a frame memory for writing and one of the two frame memories previously being used for writing is now used for reading. It is important that after each cyclic change a newly arriving data frame of each channel is always written to the frame memory which was in a read state in the previous mode.
In the above described aspect of the invention, data frames of a plurality of channels are present in one frame memory having a read-state and this frame memory is read out starting with the occurrence of the common synchronization clock. One possibility is to store one data frame in one row and also to read out the data frame synchronized to the common synchronization clock along the row direction. However, in many communication systems, on the transmitter side a bit-interleaving has been performed. In order to supply the time-aligned frames in the correct de-interleaved format, a de-interleaving must be performed in the receiver. Advantageously this can be carried out in the present invention by writing to the frame memories in the row-direction and reading-out the frame memories in the column-direction. Therefore, time-alignment and de-interleaving can be performed without increased effort in terms of hardware.
When the time-alignment apparatus, the receiver and the method are used in a telecommunication system using a convolutional coding/decoding, for example with a coding rate of r=xc2xd or r=⅓, as well as a soft-output equalizer, then the data information present in the data frames will be occurring as 2-3 symbols scrambled according to the interleaver, each with e.g. 4 soft-decision bits. Advantageously, each symbol is stored in one cell of the memory.
Further advantageous embodiments and improvements of the invention can be taken from the following description and the dependent claims. Hereinafter, the invention will be explained with reference to its embodiments and with reference to the attached drawings.